This invention generically relates to a magnetic resonance inspection apparatus. More specifically, the present invention relates to an external trigger imaging method and apparatus in a magnetic resonance inspection apparatus which will be suitable for the preparation of a plurality of slice images by continuous imaging, and which can facilitate breath holding scan imaging for reducing body motion-induced artifacts.
An ordinary imaging process in a conventional magnetic resonance inspection apparatus involves the steps of determining an inspection sectional plane or planes (slice plane(s)) of a subject, setting a sequence condition, starting a data fetch sequence by a scan start command by an operator and forming various images. In this case, imaging is generally carried out continuously by gradually changing the slice position of the subject and a plurality of sectional plane images are formed. This method is referred to as a "plurality-of-slices imaging".
Conventionally, the time required for imaging one slice image, for example, has been longer in the magnetic resonance inspection apparatus than in an X-ray CT, or the like. Particularly when those portions of a patient which move with his breathing such as the abdomen are to be imaged, therefore, body motion of the patient occurs unavoidably during the imaging process for a long time, and artifacts which result from breathing motion have been a serious problem. As to this problem, high speed imaging has been achieved in recent years and breath holding imaging has become possible with the reduction of the imaging time by the patient cooperation of holding his breath during imaging. Thus, the reduction of the artifacts have become technically possible.
An example of the timing chart of the conventional sequence for imaging a plurality of slice images is shown in FIG. 1 and the problems with the conventional method will be explained with reference to this timing chart. In the imaging process for imaging a plurality of slice images in the timing chart shown in FIG. 1, setting is made to image six slice images, for example. As other conditions, a scan parameter TR or in other words, the period of an RF (Radio Frequency) pulse is set to 240 ms, a data collection matrix is set to 256.times.256 and the number of times of integration of imaging under the same condition for improving an S/N ratio is set to once. The conventional process for imaging a plurality of slice images is constituted so as to sequentially obtain imaging data from slices 1 to 6 by setting a scan parameter TR to 240 ms.
In other words, since the data are measured in the sequence of 1, 2, 3, . . . , n as shown in FIG. 2, the scan time of 61.5 seconds is necessary to obtain the data for forming the images of the slice 1 to 6 by the calculation 0.24.times.256.times.1. This time is the inspection time required for the patient. In this manner, the conventional imaging sequence method needs a long inspection time and breath holding imaging of an ordinary patient is not practically possible. This problem cannot be solved so easily even if the imaging time can be shortened to certain extents.
In the imaging method in the conventional magnetic resonance inspection apparatus, the imaging sequence is operated by the "scan start" instruction of the operator. In this case, even if breath holding imaging becomes possible by high speed imaging, the breath holding timing must be given by the operator to the patient due to the structural limitation of the conventional apparatus. Since the magnetic resonance inspection apparatus operates in accordance with the control sequence that is in advance set for imaging, on the other hand, it is extremely difficult to establish synchronization between the instruction of the operator and the operation of the apparatus, and to time the breath holding timing.
The use of a contrast medium for magnetic resonance imaging has been permitted recently and imaging by the use of the contrast medium has been carried out to improve the contrast inside a tissue having a small difference of signal parameters. In such an imaging process, imaging must be carried out a plurality of times at certain intervals after the administration of the contrast medium so as to measure the changes of contrast due to the contrast medium with the passage of time. In this case, the imaging condition for each imaging sequence must be set whenever the predetermined time elapses and this increases the burden of the operator.
When a plurality of slice images are to be taken, the repetition time of each sequence is set in accordance with the number of slices, so that the imaging time gets elongated and breath holding imaging becomes practically impossible.
When one sectional image is taken by high speed imaging as shown in FIG. 3, pre-processing sequence (such as the optimization of the application of a gradient magnetic field, the RF irradiation system and the RF reception system) must be incorporated before the imaging sequence in the high speed imaging process by the conventional magnetic resonance inspection apparatus. If breath holding scan imaging is effected in this case, the patient cannot easily catch the optimum timing for breath holding but moves during the imaging sequence.